Method and apparatus for cold starting a steam turbine

ABSTRACT

The disclosure relates to a method and apparatus for cold starting a steam turbine by preheating a steam turbine component using eddy currents. The process includes providing an electrically conducting steam turbine component, an electro-magnetic coil, and a supply of AC to the coil. The coil is located relative to the component so that the coil is capable of forming eddy currents in the component. In this location, an AC current is passed through the coil thus heating the component.

RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 to European Patent Application No. 10 161 772.8 filed in Europe on May 3, 2010, the entire content of which is hereby incorporated by reference in its entirety.

FIELD

The disclosure relates to a method and apparatus for cold starting a steam turbine and to a pre-heating step of a starting process.

BACKGROUND

Preheating of live steam valves in ultra super critical pressure power generation (USC) plants is desirable in order to limit the thermal stress of the valve's housing.

For a cold start, the difference between valve housing temperature and saturated temperature at turbine start-up can be in excess of 280° C. This temperature difference defines a cold start. Without pre-heat, the application of live steam to the valve during a cold start can lead to rapid condensation heating of an inner surface of the valve housing which can result in high thermal stresses. Such high stress can dramatically reduce the life of the valve. This situation can be overcome by pre-warming the valve to minimise the difference between saturated steam temperature and valve housing temperature during cold start-up.

Known methods of preheating can involve electrical radiators/blankets or condensate applied to an outer surface of a steam turbine component such as a valve. These methods can have a disadvantage that applying heat only to the surface of the valve can result in a preheating temperature gradient across the entire valve. To avoid excessive stress during the pre-heat that can be created by this gradient, the preheating rate can be limited. Due to this, the heat-up rate can become a time limiting factor in the cold starting of a steam turbine.

There is therefore a need to provide a method and apparatus for preheating a valve or component that is susceptible to excessive stress during cold start of a steam turbine in as short a time as possible while minimising the detrimental effect of heat stress.

SUMMARY

A method is disclosed for preheating a steam turbine component which includes providing an electro-magnetic coil; locating the coil relative to the steam turbine component so that the coil forms eddy currents in the steam turbine component when AC is applied to the coil; and providing AC to the coil so as to heat the steam turbine component by the eddy currents.

An apparatus is disclosed for preheating a steam turbine component, including an electro-magnetic coil for arrangement relative to a steam turbine component so that the coil will form eddy currents in the steam turbine component when AC is applied to the coil; a temperature measurement device for measuring a temperature of the steam turbine component; and a controller for controlling an AC supply to the coil based on temperature of the steam turbine component.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is explained in more detail below with the aid of exemplary embodiments in conjunction with the drawings. All elements that are not essential for directly understanding the disclosure have been omitted. Identical elements are provided with identical reference numerals in the various figures. The flow direction of the media is specified by arrows. In the drawings:

FIG. 1 is a perspective view of a component with an exemplary heating system; and

FIG. 2 is a block diagram of the heating system of FIG. 1 applied to a steam valve of a steam turbine.

DETAILED DESCRIPTION

According to an exemplary embodiment of the disclosure, heating of a steam turbine component takes place by eddy currents that act within the component. Internal heating, for a given energy input, can produce relatively smaller internal temperature gradients. This makes it possible to increase the energy input and remain below specified (e.g. critical) thermal stress levels. As a result, preheating time can be reduced. An exemplary advantage is that the coils do not need to make direct contact with the component and so they are not heated to the same extent as the component. This makes it possible to locate the coils on top of thermal insulation protecting the component. This can provide improved personal safety, and can simplify installation.

In an exemplary embodiment according to the disclosure, a heat-up process is applied to a steam turbine valve. The method is useful for valves due to their complicated geometries and relatively thick walls. These factors make valves susceptible to non-uniform temperature distribution. By internally heating the valve, the effect of complicated geometry can be partially reduced and so the component can be more evenly preheated. It is thus possible to achieve a higher heat-up rate. The method can be applied to other steam turbine components, such as pipe joints, that can be susceptible to heat stress due to their geometry.

FIG. 1 shows an exemplary embodiment of a steam turbine component 10 with a heating system. The purpose of the heating system is to preheat the component 10 to bring it up to the temperature specified to, for example, start or restart the steam turbine 20 while limiting, to acceptable levels, heat stress in the component 10.

The steam turbine component 10 is, in an exemplary embodiment, made of an electric conducting material. This makes the component 10 conducive to eddy current heating.

The heating system for the heating of the component 10, in an exemplary embodiment, includes an electro-magnetic coil 18 and an AC supply 12. Although eddy current systems can provide highly predictable heating, an exemplary embodiment includes a controller 14. A component temperature measurement device 16 can be included as part of the control system.

The electro-magnetic coil 18 is arranged to induce an eddy current in the component 10. In an exemplary embodiment, this can be achieved by the coil 18 being coiled around the outer surface of the component 10, as shown in FIG. 1, without making contact with the component.

An AC supply 12, in an exemplary embodiment shown in FIG. 1, is in electrical communication with the coil 18. The AC supply 12, in an exemplary embodiment, varies the AC in the coil at a determined current and frequency suitable for heating the component 10 at a predetermined or defined rate and depth.

In an exemplary embodiment, shown in FIG. 1, the varying of the AC can be performed by a controller 14. In a further exemplary embodiment, the controller 14 includes a temperature measurement device 16 for measuring the temperature of the component 10. This enables better control of the heating rate and so permits a closer approach to the maximum allowable thermal stress limit.

An exemplary steam turbine cold start-up process includes providing a steam turbine 20, and an electro-magnetic coil 18, as shown in FIG. 2. The coil 18 can be located relative to a steam turbine component 10, 10 a so as to enable preheating of the component 10 by eddy currents. The component 10 may be a valve 10 a, including, as shown in FIG. 2, a steam valve 10 a arranged to control motive steam into the steam turbine 20, a part thereof, a piping component 10, or parts of the casing of the steam turbine 20 (not shown). To realise the heating of the component 10, AC is provided to the coil 18.

The heating, in an exemplary embodiment, is controlled by varying the AC. In a further exemplary embodiment this heating is made in response to a further measured variable, such as a temperature measurement of the component 10.

Thus, it will be appreciated by those skilled in the art that the present disclosure can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the disclosure is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.

REFERENCE SIGNS LIST

-   10, 10 a Component, steam valve -   12 AC supply -   14 Controller -   16 Temperature measurement device -   18 Coil -   20 Steam turbine 

1. A method for preheating a steam turbine component comprising: providing an electro-magnetic coil; locating the coil relative to the steam turbine component so that the coil forms eddy currents in the steam turbine component when AC is applied to the coil; providing AC to the coil so as to heat the steam turbine component by the eddy currents.
 2. The process of claim 1 wherein the steam turbine component is a steam valve.
 3. The process of claim 2 wherein the steam valve controls a flow of motive steam into the steam turbine.
 4. The process of claim 1, comprising: controlling heating of the steam turbine component by varying AC power and frequency applied to the coil.
 5. The process of claim 4, comprising: measuring temperature of the steam turbine component; and varying the AC based on the measured temperature.
 6. The process of claim 2, comprising: controlling heating of the steam turbine component by varying AC power and frequency applied to the coil.
 7. The process of claim 3, comprising: controlling heating of the steam turbine component by varying AC power and frequency applied to the coil.
 8. An apparatus for preheating a steam turbine component, comprising: an electro-magnetic coil for arrangement relative to a steam turbine component so that the coil will form eddy currents in the steam turbine component when AC is applied to the coil; a temperature measurement device for measuring a temperature of the steam turbine component; and a controller for controlling an AC supply to the coil based on temperature of the steam turbine component.
 9. The apparatus of claim 8, wherein the steam turbine component is a steam valve.
 10. The apparatus of claim 9, wherein the steam valve controls a flow of motive steam into the steam turbine.
 11. The apparatus of claim 8, comprising: insulation covering the steam turbine component. 